As a known heating apparatus of this type, there is a U.S. Pat. No. 4,370,115 (equivalent to British Patent No. 2,079,662, French Patent No. 80 15930 and German Patent No. 30 27 698) proposed by the inventor, et al.
Said conventional apparatus, however, has the following shortcomings: firstly, it is designed to cut off the heating power a little before discharge termination of the raw material and to keep this raw material flowing until the time of discharge termination. The object of this is, on the one hand, to cool and diffuse high temperature storage heat remaining in the heating tubes and connected raw material passages which are used as heating elements by means of the incessantly flowing raw material and, on the other hand, to enable the raw material itself to be kept, when its flowing stops, in a safe temperature range where the promotion of reactions can be neglected. If cooling and diffusion of storage heat are insufficient, reactions such as crosslinking and vulcanization may be irreversibly promoted in the raw material after stopping its flow, likely to make the following molding impossible. In the conventional apparatus, however, it is not only the cooling and diffusion efficiency of storage heat caused by the passing raw material after cutting off heating power which are insufficient. There is also the generation of frictional heat resulting from the raw material passing through the heating tubes which continues until the raw material stops flowing, thus rendering cooling and diffusion of said storage heat wholly insufficient. Therefore, it is in fact desirable to cut off heating power a little before, a few seconds for example, the moment of discharge termination. Actually, however, unless the heating power is cut off considerably earlier than said desirable time, thereby allowing a corresponding prolongation of the cooling time caused by the passing raw material, the said cooling effect cannot be obtained. This means that the power flow period required for heating the raw material at each discharge time, i.e., heating capacity, decreases correspondingly, causing a large quantity of raw material which has not yet reached the desired high temperature to be injected into cavities, having an ill effect on the reaction of molded products and preventing a desirable shortening of the molding cycle.
The second problem relates to the application of heating power a little before the moment of discharge initiation. The reason for applying heating power prior to the initiation of the raw material flow is to preheat the raw material stationary in the tubes and to enable the total quantity of the raw material to be injected into cavities to approach the high temperature that rapidly promotes said reaction throughout the injection of the raw material. In said heating device, however, heating of the stationary raw material due to Joule's heat generated in the heating tubes may entail the potential danger of causing the temperature at the contact surface of the raw material in the contact area between the heating tubes and raw material, to unlimitedly increase with heating time. This limits heating by applying power a little prior to discharge to a short period of only a few seconds, thus making it impossible to heat the stationary raw material all the way to its interior up to the desired temperature.
As stated above, during the molding process, the conventional apparatus takes care of the problem of heating response and has here proved to be efficient; in reality, however, it has posed a problem of inferior cooling response.